The transmission of information across a communications channel requires that the user (or source) information be converted to the form of a signal which is compatible with transmission characteristics of the communications channel. Such conversion is in many cases accomplished through modulation of an electrical or optical carrier signal, or both, by the information content itself or by compound modulation--that electrical or optical signal being transmitted over the communications channel. In the case of information in digital form, the "1" s and "0" s representing the data are encoded into a form that can be used to drive, or modulate an aspect of the transmission carrier signal, such as amplitude, frequency, phase, or polarization.
Various modulation techniques for digital data are known in the art, including on-off keying (OOK), intensity modulation (IM), amplitude modulation (AM), frequency modulation (FM), phase modulation (PM) and polarization modulation. The source information may be line coded prior to any modulation. The line coded data becomes the baseband signal modulating the electrical or optical channel carrier. Typically, such line codes map the data value "1" to a high signal value defined by the coding format and the value "0" to a low signal value. While, these line coding formats can include negative (less than zero) signal values for the low signal state, such a negative low signal state cannot be realized for optical transmission systems--the concept of "negative" light not being realizable, at least in practice. Accordingly, as the use of optical transmission media has become increasingly the norm, "unipolar" line codes have been developed in which the low signal state is maintained at zero or a small non-zero signal level.
Two line coding methods of particular interest are designated "unipolar return-to-zero" (URZ) and "unipolar non-return-to-zero" (UNRZ) coding formats. URZ and UNRZ coding are types of on-off-keying modulation well known in the communications arts. UNRZ coding is the common coding technique for transmitting data in optical communications systems.
In optical communications systems, the optical carrier signal is normally provided by a laser light source, such as a laser diode or an LED (light emitting diode). The optical output of the laser diode is typically modulated with the UNRZ-coded baseband signal, representing the coded source information, and this modulated light-wave signal will be transmitted across an optical transmission medium, such as a fiber-optic cable or free space. Modulation of the laser output may be either direct, by varying the laser diode current in proportion to the modulating signal, or indirect, through use of an external modulator. At the receiver end of the optical transmission path, an optical demodulator is applied to recover the coded baseband signal, which signal is then decoded to recover the transmitted source information.
In practice, optical communications systems include unwanted signal and/or optical energy which interferes in one way or another with a data signal being transmitted via that optical transmission system. Among the sources of such unwanted signal or optical energy are:
(1) Unwanted background, or residual light, which may be characterized as CDC (continuous DC) light or slow-varying signal energy; PA1 (2) Amplified spontaneous emission (ASE) occurring at optical amplifiers in the transmission path of an optical communications system; and PA1 (3) Slow-varying signal energy which is purposely inserted into the optical communications system independently of the data signal transmitted via that system.
The ultimate effect of the unwanted optical and signal energies is to create a bias in the data signal received at a receiving location, which basis may lead to as in the interpretation of information bits transmitted across the optical communications system. It is noted that the impact on data signal recovery from the occurrence of such unwanted optical and signal energies is significantly magnified in optical communications systems using wave division multiplexing (WDM) or dense wave division multiplexing (DWDM)
FIG. 1 provides a graphical illustration of the bias created by each of the identified unwanted optical and signal energies for a received bit or a small number of bits. In particular, the waveform at the left-most portion of the error signal waveform in FIG. 1, identified as DC Light, the bias which might be attributable to CDC light ((1) above) is illustrated. As will be seen in the figure, that bias manifests itself as substantially a constant DC level in the data signal path. In the middle portion of the error waveform, identified as ASE, the bias typically seen as a result of ASE in the optical amplifiers ((2) above) is illustrated. While, as the figure shows, the amplitude of the bias level varies over time, for a given data bit, the amplitude is substantially constant. The bias associated with purposely introduced slow-varying signal energy ((3) above) is illustrated in the right side of the error waveform in FIG. 1 and identified as Low Frequency Added Signal. That bias, as shown in the figure, will typically provide a bias at one level (approximately constant) during the first half of the period of the slowly varying signal, and at a second level during the second half of the period for such a signal. However, because the period of the slowly varying signal is much greater than the period of the data signal, the effect is that of an essentially constant bias over many bits of the data signal.
Although receivers are known in the art which are designed to reduce the error-causing impact of the above-described biases in the received signal, such receivers are not only highly complex, and thus very expensive, they are not fully effective in lighting the areas due to such biases. Accordingly, there is a need in the art for an optical transmission system which provides a consistently high level of error reduction from unwanted optical and signal energy in the transmission path of such a system, and without undue complexity.